In Wide-band Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE) systems, discontinuous reception and transmission (DRX/DTX) allows a mobile terminal, known as user equipment (UE), to switch off its radio transceiver circuitry and thereby drastically reduce its power consumption. For instance, in a WCDMA network, when a UE is idle, or is in a Cell_PCH or URA_PCH state in connected mode, it can reduce its power consumption from 100-400 mA down to 5-10 mA using DRX.
When the UE is active in the sense that it “continuously” transmits and receives packets, discontinuous transmission and reception considerably reduces the power consumption and thereby increases the active mode time or talk time (i.e. the time that the UE can stay active without needing to recharge its battery). Indeed, DRX is supported in Continuous Packet Connectivity (CPC) in WCDMA systems, which is described in 3GPP specification TR 25.903, entitled “Continuous Packet Connectivity (CPC) for Packet Data Users”, Release 7. Likewise, LTE specifications support DRX while the UE is in an LTE_ACTIVE state, which is described in 3GPP specification TS 36.300, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”.
When using DRX in an active mode (such as in LTE_ACTIVE), the DRX cycle length needs to be controlled so that the UE can send and/or receive data with sufficiently low latency, but still providing low power consumption. The base station associated with the UE can control the DRX cycle length using Radio Resource Control (RRC) and/or medium access control (MAC) signalling, or it can rely on the UE autonomously adjusting its DRX cycle. While in this latter case the amount of signalling over the radio interface is reduced, the network (i.e. the base stations and other nodes) must keep track of the current DRX cycle(s) of the UEs in LTE_ACTIVE. This is because the Radio Access Network (RAN) must be able to schedule a specific UE when, for instance, a burst of data needs to be delivered to the UE within a pre-defined deadline (depending on, among other aspects, the QoS requirements).
While in active mode, the UE uses voice and various (real-time, quasi-real-time and best effort) data services and thereby the packet inter-arrival (inter-sending) times dynamically change. Consequently, the times when the UE should activate and deactivate its receiver and transmitter may change frequently. In other words, the “optimum” situation is where the time instances that the UE should sleep and wake up are dynamically controlled to adapt to the subsequent packet arrival/departure instant. However for such “optimum” (i.e. fully dynamic) control, the associated control signalling over the radio interface would be prohibitive.
Therefore, because of the traffic dynamics, there is a fundamental trade-off between how close the DRX/DTX wake up time is to the ideal or optimum DRX/DTX setting and the amount of signalling required over the radio interface.
This trade-off between battery consumption and the complexity (and overhead) of the associated control signalling is widely recognized and several approaches have been proposed within the 3GPP.
In one approach, known as the “one level DRX/DTX cycle” approach, at any one time instant, there is a single pre-defined pattern stored in the UE according to which the UE turns its transceiver circuitry on and off. When the base station needs to change this pre-defined pattern, it uses control signalling to specify the new pattern that thereafter specifies the wake-up/sleep time instants.
In an alternative approach, known as the “two level DRX/DTX cycle” approach, two basically independent patterns are defined according to which the UE should turn its transceiver circuitry on and off. At any one point in time, by using the two patterns, the UE can calculate the next time instant when it has to change state. Alternatively, the base station can use explicit signalling to activate and de-activate either of the pre-defined cycle patterns. The two-level scheme is useful because it avoids the frequent reconfiguration of the currently used DRX pattern.
The continuous packet connectivity feature in WCDMA allows the activation of a two-level DRX scheme. In this approach, the UE switches to a longer DRX cycle if it is not served during a certain network-specified duration.
A hybrid approach that combines either the one-level or the two-level basic scheme with an autonomous behaviour of the UE has also been proposed. In this scheme, the UE dynamically and adaptively adjusts its current DRX pattern depending on the traffic arrival and departure pattern. As the UE follows pre-defined rules when adjusting its DRX interval and as the UE and the base station share a common (absolute) clock, the UE and the base station have common knowledge about the currently valid DRX interval at any point in time.
The above existing solutions focus on the trade-off between control signalling and changing or adapting the DRX/DTX cycle. Since the basic underlying assumption is that neither the UE nor the base station has enough (or accurate enough) knowledge about the traffic pattern (and thereby the packet inter-arrival times), these mechanisms are inherently reactive.
In all three basic approaches described above (one-level, two-level base station or hybrid base station controlled, and UE autonomous) the DRX/DTX cycle and the corresponding wake up and sleep times are specified and modified based on the recent activity of the UE and the base station. For instance, if the on-going service is web browsing, the UE can either autonomously, or upon an explicit order from the base station, extend its DRX period if there was no traffic activity on the radio channel. Thus the selection of the DRX cycle is mainly based on the amount of traffic in the buffer, the statistical model of the traffic type and/or a combination thereof.
As the DRX cycle is specified without precise information about the service or traffic type, the DRX pattern has to be specified without making use of information about the packet arrival/departure pattern that can be expected in the subsequent time period (which can be as much as a couple of seconds). Therefore, two problems arise:
The first problem is that the DRX cycle that the UE employs is either too conservative (the transceiver of the UE is active when there is no traffic), or it is not conservative enough (the transceiver of the UE is deactivated when there is data to schedule). In the former case (UE being too conservative) the battery lifetime is reduced, while the second case leads to increased packet delay.
The second problem is that the control signalling that is needed to keep the DRX cycle pattern up-to-date may become too frequent, which results in a high overhead and thereby wastage of radio resources.
Physical layer or MAC layer control signalling is used (for example in LTE) to switch between the DRX cycles. These channels contain a limited number of bits and their inappropriate or frequent use leads to significant overheads. Furthermore, the number of bits required increases with the increase in the number of DRX levels, (for example 1 bit for a 2-level DRX, 2 bits for up to a 4-level DRX and so on). Another limitation is that as the signalling is on/off in nature and (due to noise) it is also more prone to erroneous (or inverted) decisions at the receiver.
United States Patent Application Publication No. 2005/0032555 describes a method of intermittent activation of receiving circuitry of a mobile user terminal. The period with which the receiving circuitry is activated can be determined based on the type of service (for example packet data call, video clip, web-browsing, FTP sessions) that was provided in the last call connection to the mobile user terminal.
However, this method has the disadvantage that it relies on the most recent type of service provided to the mobile terminal to determine the cycle length, and as such, the determined cycle length might not be particularly suited to the next type of service to be provided to the mobile terminal.
Therefore, it is an object of the invention to provide improvements in the way in which a discontinuous reception and/or transmission cycle length is determined.